It is conventional in high density packaging to form circuits, subassemblies and the like on ceramic chip carriers and solder the chips to multilayer circuit board substrates. It is desirable to obtain the maximum density of circuitry on a substrate with the fewest number of processing steps. A means to accomplish this would be to utilize large chips. This approach has encountered a number of problems.
Attempts to solder large chips to conventional organic, e.g. epoxy-glass, polyimide-glass, circuit boards have not fared well due to the large temperature coefficient of expansion mismatch between the ceramic chip carriers and the substrate in the x-y direction. This produces solder joint fatigue damage during temperature and/or power cycling. Attempts to alleviate this problem, for example, by laminating circuit boards to a low expansion metal core, by using low expansion fillers in the laminates, by utilizing compliant coatings on the circuit boards and the like have not been successful for large area chip attachment.
Another approach to the problem has been the use of co-fired substates based on alumina ceramics. Such substrates require very high processing temperatures, i.e. 1400.degree.-1550.degree. C., and are therefore limited to refractory metals, such as tungsten or molybdenum, instead of significantly more conductive metals. Co-fired substrates based upon ceramic filled glass dielectrics do allow for lower processing tempeatures, but are limited to small size chips and frequently encounter interaction problems with conductor and resistor circuit materials applied between dielectric layers.
Thick film multilayer structures are advantageous in that they utilize highly conductive metallization. However, since the chips are not well matched to the substrate, they are extremely restricted in size when utilized for direct bonding.
In accordance with this invention, there is provided a devitrified glass which is suitable for use as a substrate material for the direct mounting of large silicon chips.